for an eye-movement of
40 deg. as 99.9[sigma], which is an average of only 2.49[sigma] to the
degree. Voluntary eye-movements, like other voluntary movements, can
of course be slow or fast according to conditions. After the pendulum
has been swinging for some time, so that its amplitude of movement has
fallen below the initial 47 deg. and therewith its speed past the middle
point has been diminished, the eye in its movements back and forth
between the fixation-points can still catch the after-image of _i_
perfectly distinct and not at all horizontally elongated, as it would
have to be if eye and pendulum had not moved just together. It appears
from this that certain motives are able to retard the rate of
voluntary movements of the eye, even when the distance traversed is
constant.

[19] The speed of the pendulum is measured by attaching a
tuning-fork of known vibration-rate to the pendulum, and
letting it write on smoked paper as the pendulum swings past
the 9-cm. opening.

The experiment is now as follows. The room is darkened. Card _T_ is
dropped into groove _z_, while _I_ is put in groove _y_ and swings
with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction _EA_. The pendulum is
allowed to swing through its 47 deg.. The resulting visual image is shown
in Fig. 7:1. Its shape is of course like _T_, Fig. 6, but the part _H_
is less bright than the rest because it is exposed a shorter time,
owing to the narrowness of the handle of the dumb-bell, which swings
by and mediates the exposure. Sheets of milk-glass are now dropped
into the back groove of _BB_, until the light is so tempered that
part _H_ (Fig. 7:1) is _barely but unmistakably_ visible as luminous.
The intensity actually used by the writer, relative to that of _EE_,
is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the
same amount of light would reach the retina, but that it would be
concentrated on a horizontally narrower area. And if the eye moves
exactly with the pendulum, the visual image will be no longer like 1
but like 2 (Fig. 7). We do not as yet know how the intensities of _e_,
_e_ and _h_ will relatively appear. To ascertain this we must put card
_I_ into groove _x_, and let card _T_ swing with the pendulum in
groove _y_. If the eye is again fixed in the direction _EA_ (Fig. 5),
the retina receives exactly the same stimulation that it would h